The long-term objective of this proposal is to gain insight into mechanisms of innate immunity to infectious agents in humans, with particular emphasis on the role of vitamin D in host defense. We recently reported that 25-hydroxyvitamin D (25D) is required for the Toll like receptor-induced antimicrobial response to Mycobacterium tuberculosis (M. tb.) in human macrophages including the induction of the antimicrobial peptide cathelicidin. Furthermore, African-Americans, known to have increased susceptibility to tuberculosis, had low serum 25D and their sera were inefficient in supporting cathelicidin mRNA induction unless supplemented with 25D in vitro. Our central hypothesis is that adequate host 25D primes a vigorous innate immune response to M. tb. Here, we propose to define the mechanism(s) by which TLR activation of human monocytes leads to antimicrobial activity, the role of the adaptive immune response in modulating this pathway and whether an in vivo supplementation intervention of 25D-deficient individuals with a standard regimen of oral vitamin D can restore TLR-induced antimicrobial activity in vitro. The proposed studies should provide new information about the human innate immune response with specific relevance to tuberculosis but of general interest to other microbial infections. It is hoped that the proposed studies will provide new insight into the role of 25D in innate immunity in humans and the potential use of vitamin D as an adjunct to therapy or preventative agent for infectious disease. PUBLIC HEALTH RELEVANCE: We have chosen to study tuberculosis (TB), because it is a disease of global proportions that poses a major infectious disease risk, with the pathogen killing a human being every 15 seconds. Our preliminary data suggests a role for a form of vitamin D, 25-hydroxyvitamin D (25D3), in human host defense against the TB bacillus, in particular in 25D3-deficient African Americans, who are more susceptible to TB infection. The newly identified vitamin D antimicrobial pathway allows us to determine whether therapeutic intervention with vitamin D, costing a few dollars, could augment host defense and eventually be used as chemoprevention or as an adjuvant to chemotherapy.